Social bonds form the core of society. Failures or pathologies of social bonds are major sources of physiological and psychological stress and may lead to depression or illness. The proposed research will document and examine an animal model amenable to the study of the behavioral biology and physiology of social bonding. The formation of pair bonds in female prairie voles will be the subject of the proposed research. In Phase I the stimulus requirements for pair bonding will be described. In Phase II the hypothesis that oxytocin plays a major role in the initiation of pair bonding will be tested. Oxytocin is released during mating and has been implicated in the initiation of social bonds in other models. Oxytocin release during mating will be measured in serum and in the central nervous system. Injections of oxytocin and fragments of oxytocin and oxytocin antagonists will be used to assess the behavioral effects of oxytocin in this model. Social and sexual behaviors are regulated by steroid hormones. The effects of oxytocin also may be modulated by steroids such as estrogen. The interactive effects of steroid hormones and oxytocin also will be examined. Depending on the results of these studies interactions with other neurochemical systems will be studied.